Neutron-based interrogation techniques offer unique capabilities for inspection and verification for the control of special nuclear materials, explosives detection via elemental composition analysis, and radiographic imaging through shielded containers, which are relevant to homeland security. Neutron-based elemental analysis allows industrial process control in the cement and coal industries for real-time on-line elemental analysis. However, generating sufficient neutrons for such analyses is difficult. Radioactive neutron sources are currently used in industry in a variety of places, including on-line elemental analysis of mining, coal, and cement feedstocks, sub-surface scanning (e.g., soil composition analysis and landmine detection), and radiography. The traditional neutron source has been a radioisotope such as 252Cf or Am—Be. Radioisotopes are always on, require shielding, limit types of analysis (e.g., no pulsing or time-of-flight), and pose a personnel hazard during manufacturing and assembly, as well as a security hazard due to threats of so-called “dirty bombs.” Neutrons can also be generated with conventional accelerator technology but these systems have large size and power consumption requirements. Having a compact and efficient neutron generator would directly benefit many industries by solving the problems associated with radioactive isotopes while avoiding the complications of large accelerators.
Accelerator-based neutron generators use an electric field to accelerate a beam of ions into a target. Ions from the beam react with atoms in the target, and undergo nuclear reactions (including, but not limited to, nuclear fusion). With the proper choice of ion and target species, these nuclear reactions provide a source of neutrons. The ions are often, but are not limited to, an isotope of hydrogen (H)—either protium (p or 1H), deuterium (D or 2H), or tritium (T or 3H), for several reasons, including their tendency to remain in the target at least briefly before returning to gas phase. For simplicity, hydrogen may be used to refer to any of these isotopes. The fuel species can be a combination of any compounds, atoms, isotopes, nuclei, or subatomic particles that react in such a way that neutrons are released either as a direct product of the reaction, through further decay of the reaction products, or by any other means.
The basic layout of a modern compact accelerator neutron source is shown in FIG. 1. The standard hardware consists of: (1) A high-voltage generator (˜100 kV is a common accelerating voltage) 105; (2) A target that is composed of or contains one or more of the reactants in the neutron producing nuclear reaction (e.g., titanium containing D or T) 110; (3) One or more accelerator grids; (4) An ion source assembly 115; (5) A gas-control reservoir that often uses a getter 120. Additional hardware may include a gas pressure control 125 connected to the gas reservoir 120, a magnet 130, a source anode 135, and a source cathode 140. An exemplary neutron source may also have a high-voltage insulator 145 and accelerator lens 150.
Operation of typical neutron generators proceeds as follows. Either pure deuterium (D-D system), pure tritium (T-T system), or a deuterium-tritium (D-T system) mix of gas (up to 10 Ci of T) is introduced into the system at pressures around 10 mTorr. A plasma is generated to provide ions that are extracted out of the source region and accelerated to ˜100 keV. These ions bombard the target where they can undergo fusion reactions with other hydrogen isotopes embedded in the target. DD fusion reactions generate 2.45 MeV neutrons, TT reactions generate a continuous spectrum of neutrons above 9 MeV, and the DT reaction makes 14 MeV neutrons. The systems can be operated continuously or in pulsed operation for time-of-flight measurements.
A typical neutron generator will often have an extra electrode nearest to the target that is biased negatively with respect to the target. The function of this “electron suppression electrode” is to repel negatively charged electrons that are ejected from the target during ion bombardment. This reduces the amount of current that the power supplies must drive, therefore increasing the efficiency of the generator.
The key attribute for these systems is their compact size. Modern compact accelerator-based neutron generators are typically less than 6 inches in diameter and suitable for insertion into oil well boreholes, cross-belt analysis systems, and portable explosives detection systems. There are several major suppliers of non-radioactive neutron generators, all using this type of accelerator-target configuration. List prices range between $85,000-$350,000 with the highest cost components being the high-voltage power supply, electrical feeds, and interconnects. Lifetime is limited by the degradation of the target material and the coating of insulators with best suppliers reporting ˜1000 hours for nominal output levels of 1×106 DD n/s and 1×108 DT n/s, and replacement target units range from $5-50K each. Currently, no suppliers have cost-effective high output (>1×108 n/s) DD systems, thus forcing end-users to adopt more expensive and larger footprint linear accelerator and radiofrequency quadrupole-based systems to achieve high outputs needed for inspection and analysis.